An arterial blood gas (ABG) is the traditional method of estimating the systemic carbon dioxide tension and pH, usually for the purpose of assessing ventilation and/or acid-base status. However, the necessary sample of arterial blood can be difficult to obtain due to diminished pulses or patient movement. Diminished pulses may reflect poor peripheral circulation or low blood pressure, while patient movement is frequently caused by the pain associated with arterial puncture.

A venous blood gas (VBG) is an alternative method of estimating systemic carbon dioxide and pH that does not require arterial blood sampling. Performing a VBG rather than an ABG is particularly convenient in the intensive care unit, since most patients have a central venous catheter from which venous blood can be quickly and easily obtained.

The sites from which venous blood can be sampled, measurements that can be performed on venous blood, and correlation of venous measurements with arterial measurements are reviewed here. Other alternatives to ABGs for estimating systemic carbon dioxide and pH are also described, including end-tidal carbon dioxide and transcutaneous carbon dioxide. ABGs, capnography, and mechanisms of oxygenation are reviewed separately. (See "Arterial blood gases" and "Carbon dioxide monitoring (capnography)" and "Oxygenation and mechanisms of hypoxemia".)

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